Wastewater treatment plants commonly direct wastewater into a settling tank to remove, by gravitational settling, suspended particles that are heavier than water. Settling tanks are often circular in shape and have a depth of about ten feet. The diameter of a tank is from 35 to 100 feet and is dependent on the total amount of wastewater to be treated and the number of tanks used.
Settling tanks are also used to remove matter that is of less density than water and therefore can be seen floating on the surface of the water within the tank. This floating material is commonly referred to as scum and is comprised of one or a combination of the following substances: grease, oil, algae or low density waste material.
To accomplish removal of the floating material, skimmers are often employed to push the scum into a drain trough. Most skimmers are in the form of an elongated paddle that has one side in contact with the scum layer and the other side attached to a rotatable boom. The boom is connected to a motor that causes it to rotate about the center of the tank. As the blade rotates within the tank, it sweeps the floating scum layer outwardly toward the perimeter of the tank. To catch the scum and drain it from the tank, a rectangular drain trough is located within the tank proximate its periphery and acts as a drain inlet. The trough includes an angled scumboard or baffle plate that has one end submerged in the water and the other end atop a drain opening at a height even with or slightly above the water level within the tank. As the scum is pushed outwardly by the skimmer, it moves into the trough, passes over the elevated end of the scum board and falls into the drain.
Skimmers are widely used even though there are a number of problems associated with their use. They are expensive to operate since they are electrically powered and must operate continually. There is a considerable amount of maintenance that must be performed on the skimmers because they are adversely affected by both the weather and by the relatively harsh liquids contained within the tanks. A skimmer also lacks a high degree of efficiency since the scum often moves around the drain trough without going in. Finally, in areas that experience at least occasional cold temperatures, a skimmer becomes sluggish and bogged down as it moves through the increasingly viscous liquid. If the skimmer should become covered with ice or snow, it becomes either totally inoperable or almost completely ineffective.